The present invention relates generally to fluid infusion systems, and more particularly to an improved flow metering apparatus for such systems.
The infusion of fluids into the human body is usually accomplished by means of an administration set and metering apparatus which controls the rate of flow of fluid through the set. Cassette-type metering apparatus, wherein a disposable cassette pumping element is integrally included in the administration set, and actuated by external apparatus when in use, has proven particularly attractive for this purpose because it is capable of providing a very precise control of fluid flow through the system, but does not introduce the possibility of leakage or contamination.
One form of cassette in wide use in fluid infusion systems utilizes a syringe-type pumping element in conjunction with a two-position valve which connects the metering chamber of the syringe to either the fluid source or the needle adaptor of the system. In use, the valve connects the metering chamber to the source while the plunger of the syringe is being retracted (fill stroke), and to the needle adaptor while the plunger is being advanced (pump stroke), thereby advancing fluid through the system
Typically, the actuator means for the cassette includes an actuator member which engages the plunger rod of the syringe to reciprocatively drive the plunger. Preferably, a stepping motor is used to drive the actuator member for optimum metering accuracy. Additional actuator means in the actuator apparatus engage the two position valve of the cassette so that fluid is directed into the syringe housing during the fill stroke, and toward the needle adaptor of the administration set during the pump stroke.
In many applications, such as where medication is being administered intravenously, it is necesssary that the actuation of the cassette pumping element be precisely controlled to infuse a predetermined volume of fluid at a predetermined rate. This requirement for precision control and actuation is complicated by the nature of the syringe pumping element, which because of its comparatively small volume must be filled and emptied repeatedly during the infusion process. Since pumping is not taking place while the metering chamber is being filled, to maintain metering accuracy it is necessary to compensate the delivery rate of each pump stroke for time lost in the accompanying fill stroke. This may be accomplished by increasing the delivery rate to a predetermined greater rate for a predetermined portion of the pump stroke so that the average flow rate at the end of each fill-pump cycle is the desired infusion rate.
The present invention provides an improved apparatus and method for syringe-type cassette pumps wherein compensation for the fill stroke is provided by compensating periods of increased pumping rate at both the beginning and end of each pump stroke. This provides a more uniform average pumping rate throughout the fill-pump cycle, since the higher infusion rate occurs before and after the interruption of the pump stroke by the fill stroke.
Accordingly, it is a general object of the present invention to provide a new and improved system for infusing fluids into the human body.
It is a more specific object of the present invention to provide a new and improved metering apparatus for infusing fluids into the human body.
It is another object of the present invention to provide new and improved cassette-type metering apparatus which provides a more uniform flow rate.
It is another object of the present invention to provide new and improved cassette-type metering apparatus for infusing fluids into the human body wherein compensation during the pump stroke is provided immediately preceeding and immediately following the fill stroke for improved flow continuity.
It is another object of the present invention to provide a new and improved method for infusing fluids into the human body.
It is another object of the present invention to provide a new and improved method for infusing fluids into the human body which provides a more uniform average flow rate.